The present invention relates to a hydraulic control system for an automatic transmission.
In an automatic transmission, it is the common practice to gradually engage a clutch to be applied for establishing a first forward speed ratio thereby to alleviate shocks in selecting D range from N range. Practically, an orifice is provided in a passage via which a hydraulic fluid pressure is supplied to the clutch to be engaged during operation with the first forward speed ratio, thereby to provide a gradual rise in the hydraulic fluid pressure supplied to this clutch (see a hydraulic control system disclosed in Japanese Patent Application Provisional Publication No. 55-24223 for example). If the engagement of this clutch is maintained over all of the forward speed ratios, it is sufficient to set the orifice to a size suitable for coping with shocks taking place in selecting D range from N range. However, if this clutch is to be released for establishing at least one forward speed ratio other than the first forward speed ratio, setting, as above, the orifice to the size suitable for coping with the select shocks poses another problem. In the case where there is a clutch which is kept engaged during operation with a first, a second and a third forward speed ratio, but released during operation with a fourth forward speed ratio as in the four forward speed automatic transmission disclosed in Japanese Patent Application Provisional Publication No. 56-94053, if an orifice provided in a passage via which hydraulic fluid pressure is supplied to this clutch is set to a size suitable for providing a relatively large throttle effect (i.e., an orifice with a small flow cross sectional area) to cope with select shocks, engagement of this clutch is delayed in shifting from the fourth to the third speed ratio or from the fourth to the second speed ratio, bringing about engine racing and shift shocks, thus making it very difficult to provide an appropriate shifting timing. Therefore, the conventional automatic transmission has needed a set of an orifice and an orifice control valve for coping with select shocks and another set of an orifice and and orifice control valve for coping with shift shocks taking place in shifting from the fourth to the third speed ratio and from the fourth to the second speed ratio. Besides, it is desired that the orifice for coping with select shocks should become operative only when said clutch is to be engaged (that is, a one-way orifice). This is because releasing this clutch is delayed in selecting N range from D range if this orifice is a fixed orifice, bringing about a fear that a vehicle continues to run even after selection is made to N range. As will be understood from the above description, the conventional hydraulic control system has needed the one-way orifice, the fixed orifice, and a plurality of orifice control valves for controlling them, resulting in a complicated and bulky construction and pushing up cost.